It is technically not possible to have one MRP controller for several plants.

A MRP controller is defined like storage locations too. You start with a plant and define then the controller specific to this plant.

You can have the same MRP controller number in different plants, but these are individual controllers having just the same number. Look at it from a technical point of view, the MRP controller table is T024D, it has 3 key fields Client, plant, MRP controller. In this combination it is unique. Contrary a purchasing organization, stored in T024E, it has just client and purchasing org as key fields. After the definition it can be assigned to company codes or plants in a subsequent customizing step.